water
Article
Elimination of Microplastics at Different Stages in Wastewater
Treatment Plants
Hyuk Jun Kwon 1 , Haerul Hidayaturrahman 1,2 , Shaik Gouse Peera 1, *
1
2
*
Citation: Kwon, H.J.;
Hidayaturrahman, H.; Peera, S.G.;
Lee, T.G. Elimination of Microplastics
at Different Stages in Wastewater
Treatment Plants. Water 2022, 14,
2404. https://doi.org/10.3390/
w14152404
Academic Editors: Jiangchi Fei,
and Tae Gwan Lee 1, *
Department of Environmental Science, Keimyung University, Daegu 42601, Republic of Korea;
kanata86@nate.com (H.J.K.); haerul.hidayat@gmail.com (H.H.)
National Research and Innovation Agency, Central Jakarta 10340, Indonesia
Correspondence: gouse@kmu.ac.kr (S.G.P.); wateree@kmu.ac.kr (T.G.L.)
Abstract: Microplastic pollution has been widely studied as a global issue due to increased plastic
usage and its effect on human and aquatic life. Microplastics originate from domestic and industrial
activities. Wastewater treatment plants (WWTPs) play an important role in removing a significant
amount of microplastics; otherwise, they end up in bioaccumulation. This study provides knowledge
about the characteristics of microplastics, removal efficiency, and the correlation between wastewater
quality and microplastic concentrations from three different WWTPs that differ in the type of biological and advanced wastewater treatment techniques that are believed to play an important role
in microplastic removal. Microplastics of different types, such as fragments, fibers, and beads, are
identified by using an optical microscope before and after the treatment process at each stage to assess
the effect of different treatment techniques. In the screening unit and primary clarifier unit, WWTP-B
shows the highest removal efficiency with 74.76% due to a distribution flow system installed before
the primary clarifier to ensure a constant flow of wastewater. WWTP-B uses a bioreactor consisting
of a filter plate coated with activated carbon (BSTS II) that can enhance the adaptability and adhesion
of microorganisms and showed that 91.04% of the microplastic was removed. Furthermore, only
WWTP-A and WWTP-B were applied coagulation, followed by the disc filter; they showed significant
results in microplastic removal, compared to WWTP-C, which only used a disc filter. In conclusion,
from all WWTP, WWTP-B shows good treatment series for removing microplastic in wastewater;
however, WWTP-B showed a high rate of microplastic removal; unfortunately, large amounts of
microplastics are still released into rivers.
Keywords: wastewater; microplastics; removal; treatment plant; coagulation; disc filter
Qian Zhou, Zhenxing Wang and
Lizhi Xiong
Received: 21 June 2022
Accepted: 29 July 2022
Published: 3 August 2022
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1. Introduction
Plastics are widely used in various sectors of life, especially as packaging materials.
Plastic has many advantages ranging from being cheap, durable, lightweight, and easy
to obtain; however, after being used, all plastics end up as waste that accumulates in
nature, especially in the aquatic environment [1]. The presence of plastic in water bodies
is one of */the main factors affecting water pollution because it is difficult to control. In
general, plastic waste is classified depending on size, such as megaplastics (more than
500 mm), macroplastics (50–500 mm), mesoplastics (5–50 mm), microplastics (<5 mm),
and nanoplastics (less than 0.3 mm) [2,3]. Microplastics can be grouped into primary and
secondary microplastics [4,5]. Primary microplastics are intentionally produced in micro
sizes for skincare products, textile fibers, and other industrial uses. Secondary microplastics
are generated from the degradation or breakdown process of large plastic particles [6,7].
Microplastics derived from various point sources and non-point sources are eventually
carried by rivers into water bodies, including lakes, seas, and oceans [5,8]. In the last few
years, the accumulation of microplastics in marine ecosystems has gradually increased,
and microplastics found in aquatic environments have long-lasting detrimental effects on
Water 2022, 14, 2404. https://doi.org/10.3390/w14152404
https://www.mdpi.com/journal/water
Water 2022, 14, 2404
2 of 14
marine organisms [9,10]. For example, Albano et al. [11] studied microplastic pollution in
Pelagia noctiluca in the Straits of Messina from forty-nine specimens. There were 55 fibrous
shapes of microplastics with a size between 0.09 and 9.4 mm; the authors hypothesized
that microplastics originated from direct Mauve stinger accidental ingestion and biomagnification. They also proposed the Pelagia noctiluca as a water column bioindicator for
microplastic pollution in the Mediterranean Sea. The effect of polystyrene microspheres on
the Artemia salina showed that microplastics were found in this organism after 6 h, increased
exposure to concentrations resulted in decreased growth of Artemia salina due to lack of
food sources, and mortality occurred after 96 h and 120 h after microplastic exposure [12].
Another study showed the presence of microplastics in marine fish (Sparus aurata) and
freshwater fish species (Cyprinus carpio) at different life stages from two fish farms located
in Italy and Croatia. The result shows that microplastic concentrations in Cyprinus carpio
were lower than Sparus aurata, with microfiber abundance of 0.25 and 1.3 items/individual,
respectively. Low microplastic concentration in these fish species depends on the concentration of microplastics in water and the presence of microplastics in feed aquaculture [13].
Microplastics are ingested by aquatic organisms and transported through the water bodies
via the food chain, eventually causing irreparable damage to human health [14,15].
A WWTP receives wastewater containing high amounts of microplastics every day. Microplastics entering WWTP come from domestic, industrial, and agricultural wastewater [1].
Various studies explored the behavior of microplastics during the treatment process, and
the results obtained depend on the type of wastewater and technology used. Microplastics
of large size and low density are easily removed by physical treatment through screening,
grit removal, and sedimentation processes in the primary clarifier, whereas large-shaped
microplastics, such as fibers and fragments, are eliminated by floatation. On the other hand,
high-density microbeads generally sink to the bottom of the sedimentation tank due to
gravitational force. The physical treatment process aims to remove not only large debris
particles but also large microplastics in order to maintain the performance of the facility for
the subsequent treatment process [4,16,17]. Furthermore, smaller microplastic particles are
bound with microorganisms in the form of flocs and deposited in the biological treatment
and secondary clarifier [18,19]. Previous research showed that 90–98% of microplastics
in wastewater could be removed after going through several processing stages and using
advanced technology such as disc filter, rapid sand filtration, dissolved air floatation, or
ozonation [16,19–21]. Most prior studies were focused on the concentration of microplastics
in the effluent and then concluded WWTP as a source in the aquatic environment [22,23].
Another potential threat of microplastics is recycled activated sludge. Sludge is widely
used as fertilizer in agriculture or as a raw material for concrete and finally becomes a part
of pollutants in terrestrial [24].
In this study, determination of the type of microplastic polymer will not be conducted;
information regarding the removal of microplastics during the wastewater treatment
process was focused on investigating the abundance and shape of microplastics in three
full-scale WWTP and evaluating the removal rate of microplastics at different treatment
stages. Finally, the number of microplastics in treated water was calculated as the emission
of microplastic pollution from WWTP to the nearby river. This study provides insight into
which stage of treatment is most useful for removing microplastics significantly.
2. Materials and Methods
2.1. Sampling Sites
This study was conducted at three full-scale WWTPs in Gyeongsangbuk-do Province,
Republic of Korea. All WWTPs have a similar process from beginning to end, with some
differences in the technology used (Table 1). Firstly, the physical process will treat wastewater entering the WWTP in bar screening, grit removal, and primary clarifier for separating
solid organic matter from wastewater. Secondly, the biological process will continue to
treat water from the previous process by using microorganisms that play a role in removing organic pollutants and small particles by binding or absorbing these pollutants into
settled in the secondary clarifier. Furthermore, the effluent of the secondary clarifier then
flows into the coagulation process to remove total phosphorus, followed by a disc filter to
improve water quality in the wastewater treatment process before discharge into the river.
Water 2022, 14, 2404
3 of 14
Table 1. Characteristics of WWTPs.
Source of Capacity
Physical
Biological
Advanced
3/day)
Wastewater
(m
Treatment
Treatment
Treatment
flocs. Next, flocs of microorganisms and any remaining organic sediment are settled in the
1 and
secondary clarifier. Furthermore, the
effluent
of the
secondary
clarifier
then flows into the
Grit
removal
and
TEC-BNR
Coagulation
Industrial
coagulation
process
to
remove
total
phosphorus,
followed
by
a
disc
filter
to improve
water and
80,000 rectangular sedi- circular clarificaA
disc filter
quality inDomestic
the wastewater treatment process before discharge into the river.
mentation tank
tion
Table 1. Characteristics of WWTPs. Grit removal and BSTS II 2 and cir- Coagulation and
B
Domestic
26,000 rectangular sedicular clarificationAdvanced
disc filter
Capacity
Physical
Biological
mentation tank
(m3 /day)
Treatment
Treatment
Treatment
Grit removal
and IFAS 3 and rectanGrit removal and rectangular
TEC-BNR 1 and circular
80,000
Coagulation and disc filter
clarification
gular sedimentaDisc filter
C
Domestic sedimentation
13,000 tank
rectangular sedi2
Grit removal and rectangular
BSTS
II and circular
tion tank
mentation
tank
26,000
Coagulation and disc filter
WWTP
WWTP
Source of
Wastewater
A
Industrial
Domestic
B
Domestic
sedimentation tank
C
clarification
TEC-BNR: Taeyoung External Carbon for Biological Nutrient Removal. 2 BSTS-II: BioMecca Sew3 and rectangular
Grit removal and rectangular
Integrated
Fixed film Activated
Treatment
System-II. 3 IFAS:IFAS
Domestic age & Wastewater
13,000
DiscSludge.
filter
sedimentation tank
sedimentation tank
1
1
TEC-BNR: Taeyoung External Carbon for Biological Nutrient Removal. 2 BSTS-II: BioMecca Sewage & Wastewa-
2.2.terSample
Collection
Treatment
System-II. 3 IFAS: Integrated Fixed film Activated Sludge.
Three days were randomly selected during the Autumn (October to November 2019)
2.2. Sample
Collection
to collect
samples
and reduce the effect of algae (Table S1) [25]. In order to study the redays were
randomly
selectedprocess
during the
(Octoberthe
to November
2019) to
moval Three
efficiency
of each
treatment
in Autumn
the WWTPs,
microplastics
in raw
collect
samples
and
reduce
the
effect
of
algae
(Table
S1)
[25].
In
order
to
study
the
removal
wastewater and the effluent water of each technological step were sampled. As shown in
efficiency of each treatment process in the WWTPs, the microplastics in raw wastewater
Figure
1, water samples were collected at raw wastewater influent (W1), effluent water of
and the effluent water of each technological step were sampled. As shown in Figure 1,
primary clarifier (W2), effluent water of secondary clarifier (W3), effluent water of coaguwater samples were collected at raw wastewater influent (W1), effluent water of primary
lation
(W4),
andeffluent
disc filter
outlet
or finalclarifier
effluent(W3),
water
(W5).water
Twoofliters
of water
samples
clarifier
(W2),
water
of secondary
effluent
coagulation
(W4),
were
eachortreatment
sampling
point
[16].
The
grab samples
sampling
method
was seandcollected
disc filter at
outlet
final effluent
water (W5).
Two
liters
of water
were
collected
lected
for
sample
collection,
utilizing
a
custom-made
sampler.
The
sampler
was
at each treatment sampling point [16]. The grab sampling method was selected for sample concollection,
a custom-made
sampler.
The as
sampler
constructed
a stainlessstructed
of autilizing
stainless-steel
container
with rope
a toolwas
to take
a waterofsample
from apsteel container
with
rope as
tool to take aand
water
sample
from approximately
30 cm transferred
depth
proximately
30 cm
depth
of awastewater
sludge
stream.
All samples were
of wastewater
sludge
stream.
samples
were transferred
to the laboratory and stored
to the
laboratoryand
and
stored
at 4 OAll
C until
further
processing.
at 4 ◦ C until further processing.
Figure 1. Sampling points and typical wastewater treatment process.
Figure 1. Sampling points and typical wastewater treatment process.
2.3. Extraction of Microplastic
2.3. Extraction
of Microplastic
Several steps
were taken to reduce the incidence of microplastic contamination. NaturalSeveral
fabric cloth
was
worn
underneath
thethe
clean
white lab
All laboratory
equipment Natsteps
were
taken
to reduce
incidence
ofcoat.
microplastic
contamination.
was
cleaned
three
distilled Hthe
beforewhite
use, and
work
was wiped
2 O clean
ural
fabric
cloth
wastimes
wornwith
underneath
labthe
coat.
Allsurface
laboratory
equipment
down with 70% ethanol (Duksan, Pure Chemical, Kyungkido, Korea). Wet digestion was
was cleaned three times with distilled H2O before use, and the work surface was wiped
applied to remove organic matter from wastewater (W1–W3) [18,26] using 30% hydrogen
down
with(H
70%
ethanol (Duksan, Pure Chemical, Kyungkido, Korea). Wet digestion was
peroxide
2 O2 ) (Santoku Chemical Industry, Sendai, Japan) until the sample became clear.
applied
to
remove
organic
matter
wastewateris(W1–W3)
[18,26]
hydrogen
For the other samples
(W4 and
W5),from
no pre-treatment
needed because
theusing
water30%
sample
is
peroxide
(Hclear.
2O2) After
(Santoku
Chemical
until using
the sample
became
relatively
pre-treatment,
allIndustry,
wastewaterSendai,
samplesJapan)
were filtered
a glass microfiber filter with a 1.2 µm pore size (Whatman GF/C) (GE Healthcare, Buckinghamshire,
UK) and 30% hydrogen peroxide (H2 O2 ) added to the remaining samples the filter paper
to make sure no organic material in the sample water. Adding H2 O2 in post-treatment will
also change the physical properties of the natural fiber surface. Finally, filter papers were
transferred to glass Petri-dishes for visual analysis using a digital microscope.
filter papers were transferred to glass Petri-dishes for visual analysis using a digital microscope.
2.4. Inspection and Identification of Microplastics
Water 2022, 14, 2404
4 of 14(Leica
Microplastics in filter papers were visually inspected with a digital microscope
DM 750 combined with camera), 100× magnification, and light projected from above. The
IMT iSolution Lite Version 22.1. is used to take photographs and measure microplastic
2.4. Inspection
Identification
of Microplastics
dimensions.
Alland
suspected
microplastics
were counted and categorized by shape (fragMicroplastics
in filter papers
were visually
inspected
with
digital when
microscope
ment, fiber,
and microbead).
The following
details
should
bea noted
using(Leica
an optical
DM 750 combined
with camera),
100× magnification,
and
light
projectedcolor
fromdistribution,
above. The (2)
microscope
for microplastics
identification:
(1) clear
and
uniform
iSolution
Version
22.1.(3)
is no
used
to take
photographs
and
measure
microplastic
theIMT
object
has noLite
metallic
luster,
visible
tissue
or natural
organic
structures
attached
dimensions.
All
suspected
microplastics
were
counted
and
categorized
by
shape
(fragment,fiber,
to it, (4) same dimensions throughout the entire length and thickness of the synthetic
fiber, and microbead). The following details should be noted when using an optical
(5) there are twists (convolutions), striations, zigzags, or jagged and irregular widths along
microscope for microplastics identification: (1) clear and uniform color distribution, (2) the
theobject
natural
and luster,
(6) by (3)
adjusting
the
angle
incidence
ofstructures
light andattached
the brightness,
hasfibers,
no metallic
no visible
tissue
or of
natural
organic
to
theit,edges
of
the
transparent
microplastics
will
be
seen
[18,27,28].
(4) same dimensions throughout the entire length and thickness of the synthetic fiber,
(5) there are twists (convolutions), striations, zigzags, or jagged and irregular widths along
2.5.the
Statistical
Analysis
natural fibers,
and (6) by adjusting the angle of incidence of light and the brightness,
theInedges
of the transparent
microplastics
will be seen
[18,27,28].
situ wastewater
quality
data are collected
from
each process at the same point and
day.
2.5.Statistical
Statistical analysis
Analysis was used to find a correlation between water quality and the number of microplastic. Furthermore, this correlation was analyzed by multiple comparisons
In situ wastewater quality data are collected from each process at the same point and
with
a
significance
levelwas
set used
at 0.05.
IBMa correlation
SPSS Statistics
version
wasand
used
statistical
day. Statistical analysis
to find
between
water24.0
quality
the for
number
analysis
and
for
generating
some
graphs.
of microplastic. Furthermore, this correlation was analyzed by multiple comparisons with
a significance level set at 0.05. IBM SPSS Statistics version 24.0 was used for statistical
3. Results
Discussion
analysis and
and for
generating some graphs.
3.1.3.Overall
Microplastic Removal
Results and Discussion
wastewater
3.1.WWTP
Overalltreats
Microplastic
Removalcontaining significant amounts of microplastics from various sources
(domestic,
industrial
activities)
before amounts
being discharged
into nearby
rivers.
WWTP treats wastewater containing
significant
of microplastics
from varWWTP
can remove
a significant
of before
microplastics,
and it can
benearby
seen inrivers.
Figure S1
ious sources
(domestic,
industrialamount
activities)
being discharged
into
WWTP
can remove
a significantdecreased
amount of microplastics,
andinfluent
it can be to
seen
in Figure
S1
that
microplastic
concentration
drastically from
effluent;
however,
that
microplastic
concentration
decreased
drastically
from
influent
to
effluent;
however,
microplastics are still released into aquatic ecosystems in a definite amount. All WWTP
microplastics
are still
released into aquatic
ecosystems
in a definite
amount.
All
WWTPfound
show
a good result
of microplastic
removal
(Figure 2A).
Microplastics
are
always
show a good
result of
microplastic
removal
2A). Microplastics
always found
in apin varying
amounts,
and
this is related
to (Figure
the performance
of eachare
treatment
process
varying amounts, and this is related to the performance of each treatment process applied
plied to all WWTPs. As shown in Figure S1, the range concentrations of microplastic in
to all WWTPs. As shown in Figure S1, the range concentrations of microplastic in the
theeffluent
effluent
from
three
WWTPs
were
91–175
MP/L,
higher
than
the amount
of microplastic
from
three
WWTPs
were
91–175
MP/L,
higher
than the
amount
of microplastic
in
in the
Nakdong
River
(0.29–4.76
MP/L)
as
reported
by
[29].
the Nakdong River (0.29–4.76 MP/L) as reported by [29].
Figure 2. Removal of microplastic in different treatment processes. (A) Percentage of microplastics
removed at samping point; (B) Overall percentage of microplastic removal.
Among related studies, the microplastic concentration varies from influent to effluent.
Some studies showed high microplastics concentration in influent (2223–10,044 MP/L)
and effluent (29–447 MP/L), meanwhile low microplastic concentration were also found
in influent (1 MP/L) and effluent (0.00088 MP/L) [22,30]. These results are caused by
different sampling techniques, identification methods, and technology applied in every
WWTP [23]. For example, the filter paper pore size used to capture the microplastic from
Water 2022, 14, 2404
tween microplastic emission and those factors.
In terms of shape, microplastic samples were observed under the microscope and
grouped into microbeads, fibers, and fragments (Figure 3). Based on Figure 4, the proportion of fragments was the highest in all WWTPs, with the lowest proportion of 53.63%
5 of 14
only in the influent of WWTP-A. The proportion of fragments decreased gradually, with
only 21.18% in the effluent of WWTP-A. The proportion of microbeads increased gradually in the subsequent treatment sections and reached 55.29% in the effluent of WWTP-A.
the sample is different and often varying [30–33]. Currently, no standard method for
During
the entire processing sequence, fragments and fibers were removed in large quanmicroplastic analysis exists from the beginning to the end; therefore, it is necessary to
tities.
As
a result,
the proportion
of these
two types ofSeveral
microplastics
wasasdecreased
standardize
the procedures
of research
on microplastic.
factors such
differencesin the
effluent
portion density,
while the
proportion development
of microbeads
was
increased.
in population
local/regional
area,
and
catchment area also affect
fragments
are
usually
come from
theIrregular
microplastic
emissions
in mainly
an area. secondary
Although inmicroplastics,
general, there iswhich
a positive
correlation
between
microplastic
emission
and
those
factors.
the fragmentation of larger plastic objects, such as tires, bottles, and plastic bags [34,35].
In terms
of shape,
microplastic
observed often
undercontain
the microscope
Many types
of facial
cleansers,
facialsamples
scrubs,were
or exfoliants
plastic and
particles,
grouped
into
microbeads,
fibers,
and
fragments
(Figure
3).
Based
on
Figure
4,
the
proporand they are considered to be one of the sources of microbeads [36]. Possible sources of
tion of fragments was the highest in all WWTPs, with the lowest proportion of 53.63% only
fibers in wastewater came from the laundering of synthetic fabrics and shedding of texin the influent of WWTP-A. The proportion of fragments decreased gradually, with only
tiles
during the aging process for cloth, linen, carpets, etc. [37]. The washing of synthetic
21.18% in the effluent of WWTP-A. The proportion of microbeads increased gradually in
materials
could release
a large
number
of plastic
fibers,
soeffluent
their presence
in surface
the subsequent
treatment
sections
and reached
55.29%
in the
of WWTP-A.
Duringwater
may
dueprocessing
to the inflow
of sewage,
according
to the
reported
Airborne
thebe
entire
sequence,
fragments
and fibers
were
removedstudies
in large[37].
quantities.
As contamination
of
open
wastewater
treatment
plant
systems
must
also
be
considered
a result, the proportion of these two types of microplastics was decreased in the effluentwhen
portion while
the proportion
of microbeads
was increased.
assessing
microplastic
emission
[38].
Water 2022, 14, x FOR PEER REVIEW
6 of 13
Figure
3. 3.
Result
formicroplastic
microplastic
particles.
Figure
Resultofofmicroscope
microscope observation
observation for
particles.
Figure 4.
portion
of microplastic
shape
between
influent
(W1) and
effluent
(W5). (W5).
Figure
4. Comparison
Comparison
portion
of microplastic
shape
between
influent
(W1)
and effluent
3.2. Microplastic Removal with Different Treatment Processes
3.2.1. Screening and Primary Clarifier
The main purpose of the screening process is to separate solid particles such as organic and inorganic materials from the wastewater and allow the remaining solids particle
to sink to the bottom of the primary clarifier. In general, the design for this process is
Water 2022, 14, 2404
6 of 14
Irregular fragments are mainly secondary microplastics, which usually come from the
fragmentation of larger plastic objects, such as tires, bottles, and plastic bags [34,35]. Many
types of facial cleansers, facial scrubs, or exfoliants often contain plastic particles, and they
are considered to be one of the sources of microbeads [36]. Possible sources of fibers in
wastewater came from the laundering of synthetic fabrics and shedding of textiles during
the aging process for cloth, linen, carpets, etc. [37]. The washing of synthetic materials could
release a large number of plastic fibers, so their presence in surface water may be due to the
inflow of sewage, according to the reported studies [37]. Airborne contamination of open
wastewater treatment plant systems must also be considered when assessing microplastic
emission [38].
3.2. Microplastic Removal with Different Treatment Processes
3.2.1. Screening and Primary Clarifier
The main purpose of the screening process is to separate solid particles such as organic
and inorganic materials from the wastewater and allow the remaining solids particle to sink
to the bottom of the primary clarifier. In general, the design for this process is rectangular
and has a hydraulic retention time of ~2 h. It is expected that the removal rate of the total
suspended solids will reach 50–70% [39]. In addition, by skimming and sedimentation
processes, microplastics are also expected to be removed from wastewater [22]. The flow of
wastewater also affects the effectiveness of the primary clarifier. If the flow is too fast, it will
be difficult for the solid particles to sink to the bottom of the system and vice versa [40].
During the screening and primary clarifier, the microplastic removal rate in WWTP-A
was 69.52%, followed by WWTP-B was 74.76%, and WWTP-C was 58.62 % (Figure 2B). In
terms of shape, this series of processes had a significant removal rate for fiber in the range
of 64.60–79.59% and 21.88–68.42% of fragments and microbeads, respectively (Figure 5A).
This study proved that most of the microplastic was removed during this process. WWTP-B
shows good results for eliminating microplastics due to a distribution flow system installed
before the primary clarifier and ensuring that the wastewater flow rate is the same for each
primary clarifier unit. On the other hand, WWTP-A and WWTP-C did not use this flow
system, and wastewater flow depends on the actual discharge.
The retention time is the time required for a certain amount of wastewater to pass
through a sedimentation tank at a specific flow rate. Inside the tank, the microplastic
particles in the wastewater take time to cross the sediment tank and settle at the bottom
of the tank as it flows slowly through the tank. WWTP-B has the lowest retention time,
followed by WWTP-A and C with 0.104 days, 0.107 days, and 0.256 days, respectively.
According to our data, retention time has correlated with microplastic removal; the lowest
retention time shows the highest microplastic removal. It means that when the retention
time is low, wastewater will take a long time to pass through the tank so that microplastic
attached with another particle has the opportunity to settle more at the bottom of the tank.
This parameter also controls the performance of the primary clarifier.
Microplastics in wastewater are generally suspended individually or attached to larger
particles such as paper, wood branches, or larger plastic particles. Most of the microplastics
adsorbed to these larger particles will be easily removed during the screening process [41].
The microplastics that settle during this process are microplastics attached to the sand
particles so that they settle very easily [18]. Some microplastics float on the surface of
the wastewater because they have a lower density, which can then be easily removed
by skimming.
A similar study showed that the concentration of microplastics in wastewater is
reduced significantly through the screening, skimming, and settling process [16]. The
results show a consistent trend of microplastic shape removal rate with previous studies,
in which fiber was removed due to being easily entrapped in solid floc particles during
screening and settling on a primary clarifier [18]. Hongprasith et al. [42] stated that
microplastics were suspended together with other fine particles to form suspended solids
or were mutually adsorbed between the two.
process [41]. The microplastics that settle during this process are microplastics attached to
the sand particles so that they settle very easily [18]. Some microplastics float on the surWater 2022, 14, 2404
7 of 14
face of the wastewater because they have a lower density, which can then be easily removed by skimming.
Figure
5. Removal
efficiency of shapes
microplastic
shapes
at eachprocess.
treatment
(A) Microplastic
Figure 5. Removal
efficiency
of microplastic
at each
treatment
(A)process.
Microplastic
rateand
of screening
primary
clarifier; (B) Microplastic
removal
rate of bioreactor
and
removal rate ofremoval
screening
primary and
clarifier;
(B) Microplastic
removal rate
of bioreactor
and
secondary
clarifier; (C) removal
Microplastic
rate of coagulation;
(D) Microplastic
removal
secondary clarifier;
(C) Microplastic
rateremoval
of coagulation;
(D) Microplastic
removal rate
of rate
of disc-filter.
disc-filter.
3.2.2. Bioreactor and Secondary Clarifier
A similar study showed that the concentration of microplastics in wastewater is reBioreactor treatment aims to destroy the organic material contained in the wastewater;
duced significantly
through the screening, skimming, and settling process [16]. The results
the resulting suspended particles were deposited in the secondary clarifier. The activated
show a consistent trend of microplastic shape removal rate with previous studies, in
sludge used in this process can indirectly reduce the number of microplastics in wastewater.
which fiber was
removed
due to beingfurther
easilydeclined
entrapped
in biological
solid floctreatment
particlesand
during
The number
of microplastics
during
secondary
screening andsedimentation.
settling on a primary
clarifier
[18].
Hongprasith
et
al.
[42]
stated
that
microMicroplastic removal reached 72.55–91.04% after bioreactor treatment
plastics were(Figure
suspended
other
fine
particlesshown
to form
suspended
or was
2A,B).together
Accordingwith
to the
shape
distribution
in Figure
5B, thesolids
fragment
theadsorbed
most dominant
fraction
removed in samples from all WWTPs, contributing to about
were mutually
between
the two.
87.26–93.75%. Sheets were also a dominant fraction, ranging from 55.86 to 70.00% in
WWTP-B
and WWTP-C.
3.2.2. Bioreactor and Secondary
Clarifier
Same as the previous process, WWTP-B showed a more dominant microplastic reBioreactor
treatment aims to destroy the organic material contained in the
moval rate compared to WWTP A and WWTP C. Based on Table 1, each WWTP uses
wastewater; the
resulting
suspended
were deposited
in the secondary
clarifier.
different
technologies.
BSTSparticles
II is a biological
treatment technology
that applies
the microThe activatedbial
sludge
used
in
this
process
can
indirectly
reduce
the
number
of
microplascontrol tank in the last part of the bioreactor. This tank will enhance the adaptability of
tics in wastewater.
The number
microplastics
further declined
duringactivity
biological
microorganisms
to theofwastewater
and promote
activated sludge
in thetreatbioreactor.
There is asedimentation.
filter plate in the Microplastic
tank coated with
activated
carbon,72.55–91.04%
which is usefulafter
for increasing
ment and secondary
removal
reached
bithe
adhesion
of
microorganisms
so
that
the
bioreactor
works
efficiently.
This
technology
oreactor treatment (Figure 2A,B). According to the shape distribution shown in Figure 5B,
remove
organic
matter, nutrients,
microplastic
from wastewater.
with the
the fragmentcan
was
the most
dominant
fractionand
removed
in samples
from allCoupled
WWTPs,
clarifier, increasing microplastic removal can be achieved. The CNR technology applied
Water 2022, 14, 2404
8 of 14
in WWTP-C has relatively the same process as WWTP-B by using a filter medium in the
aerobic tank to stabilize microorganisms. The low microplastic removal rate is due to
the design of the secondary sedimentation system, which is made in a rectangular shape
without any skimming process on the surface of the tank/pond. Secondary processing at
WWTP-A has the lowest microplastic removal rate compared to other WWTPs. The main
factor is the TEC-BNR technology used. The technology modifies conventional bioreactor
technology by combining activated sludge with fermentation solutions from food waste.
By adding the fermentation solution, biological degradation will increase the performance
of the bioreactor to remove organic matter and nutrients; however, during the fermentation process, the microplastic from food packages still exists even in a small portion. It
will increase the number of microplastics in the wastewater and decrease microplastic
removal efficiency in the bioreactor. The trend of microplastic removal rate in the secondary
treatment process has the same trend as BOD, COD, SS, and T-P removal at each WWTP
(Figure S2).
Microplastics were removed together with dissolved organic matter through the activity of microorganisms and sedimentation. Hongprasith et al. [42] showed that activated
sludge greatly contributes to the microplastic removal process. The hydrophobic characteristics of microplastics also help accelerate the binding process of microplastics with
organisms or sludge in biological reactors. Fragments might be trapped into sludge flocs
by the ingestion process of microorganisms as activated sludge [43].
3.2.3. Coagulation
Coagulation was designed in all WWTPs to treat total phosphorus that cannot be
completely removed from previous treatment processes; however, this process can also
remove microplastics in wastewater efficiently. The performance of microplastic removal
was investigated with different dosages of Poly Aluminum Chloride (PAC) as a coagulant
at WWTP-A and WWTP-B for ± 72 mg/L and ± 36 mg/L, respectively. Only WWTP-C
does not use coagulation to remove total phosphorus. As shown in Figure 2B, the removal
efficiency of the coagulation process in WWTP-A was 42.26% compared to WWTP-B, with
microplastic removal efficiency being 15.79%. According to this study, the low removal rate
in WWTP-B is related to the lack of interaction between the coagulant and microplastic to
generate flocs. Although the dose of coagulant used in WWTP-A shows more effectiveness
in removing microplastics, WWTP-B cannot apply this dose directly to their plant, which
requires further research.
Figure 5C shows the microplastic removal rate during coagulation. The result indicates
that WWTP-A can remove 68.75% and 74.66% of microbeads and fragments, respectively.
In WWTP-B, the highest removal rate was fiber at 23.08%. These results indicate that
all shapes of microplastic will be agglomerated into floc particles and settled down in a
sedimentation tank. Another factor regarded as important for microplastic removal in
coagulation is the surface of the microplastic. The efficiency of coagulation increased when
the plastic surface was weathered, especially fragments [44].
Similar studies of coagulation experiments using microbeads/microsphere showed
high removal efficiency >90% [45,46]. Ma et al. [47] reported that the microplastic removal
efficiency was 36.89% for 15 mmol/L (calculated as 405 mg/L) with Al-based as a coagulant.
In contradiction, Rajala et al. [46] reported 98.2% microplastic removal with Polyaluminum
Chloride at a metal dosage of 1.4 mmol/L. Wang et al. [48] also reported that during
coagulation combined with sedimentation, the microplastic removal rate was 40.5–54.5%
with high Al-based salt concentration, and 50.7–60.6% fibers were removed through this
process. The different results are mainly caused by the different dosages of coagulant,
microplastic type, and wastewater characterize used during the experiment.
Water 2022, 14, 2404
9 of 14
3.2.4. Disc Filter
The concentration of microplastics in the effluent of the disc filter represents the number of microplastics released into the river. The removal efficiency of the microplastics by
the disc filter was 43.13–72.50% (Figure 2B), and total microplastic removal for WWTP-A,
WWTP-B, and WWTP-C was 98.87%, 98.92%, and 98.10%, respectively. In all WWTP,
fiber was the most efficient removal process of microplastics during the disc filter process (52.38–81.25%), followed by microbead (43.67–62.89%) and fragment (35.71–42.35%)
(Figure 5D). Generally, as the size of microplastics determine whether they can pass through
the filter, the disc filter should have retained microplastics whose size is more than the
pore size of the filter mesh. In addition, the shape of the microplastics also needs to be
considered. The result indicates that the fragment size is larger than the other shapes, so the
disc filter process cannot remove the fragment efficiently compared to other shapes. As a
result, the proportion of fiber and microbead shape was decreased in the effluent compared
to the previous treatment.
Changing the flow rate and pressure during the disc filter process will influence the
microplastic removal rate. In this case, the pressure in all WWTPs is the same, but WWTP-B
uses a larger flow rate than WWTP-A and C. The results showed that the microplastic
removal rate in WWTP-B was the lowest (Figure 2B) due to the larger flow rate and pressure
that can damage the filter quicker than normal. The larger flow rate can reduce microplastic
removal efficiency because microplastic sizes, which are relatively larger than the fiber’s
pores, will be forced out. Although the disc filter performance can remove microplastic
in all WWTPs, microplastics are still present in the treated water in all WWTPs. This is
probably due to the maintenance of the disc filter process by activating high-pressure
backwash and some microplastics passing through the system.
Previous studies also proved that the microplastic removal rate after disc filter use was
40–98.5% [38] and advanced filtration in Germany WWTP was 93–95% [20].
Talvitie et al. [38] reported that fibers were removed efficiently during the disc filter process
and contributed 20–100% after the treatment of total microplastics. Once this process
is in progress, fiber and other shapes of microplastics can pass through the disc filter
longitudinally; even when the pore of the membrane filter is 0.08 microns in size [49].
This condition shows that the movement of microplastics can occur at WWTPs applied to
membrane technology in smaller pore sizes [38,50]. As an advanced technology, disc filters
need to be developed to become a promising technology for removing microplastics in
wastewater [38,51]. The microplastic removal rate will be affected by various technologies
applied to tertiary treatments [38,52]. Our study proved that coagulation and disc filter
treatments increase the microplastic removal rate for all WWTP.
3.3. Correlation with Wastewater Quality Data
Wastewater quality parameters are monitored regularly at all stages of treatment,
such as biochemical oxygen demand (BOD), chemical oxygen demand (COD), suspended
solids (SS), total nitrogen (T-N), and total phosphorus (T-P). Wastewater quality parameters
decrease after passing through in all treatment processes with different efficiency removal
from three WWTP (Table S2). From all WWTP, the microplastic concentration has a positive
correlation with all water quality parameters (Figure 6). Suspended solid shows a high
positive correlation with the microplastic removal at all treatment stages, followed by COD
and BOD. Suspended solids in wastewater contain microplastics and other particulate
material, and together, they can be removed through all series of treatment processes.
COD and BOD are indicators of organic content that are indirectly related to the number
of microorganisms and microplastics in water bodies. When the wastewater treatment
unit removes organic elements, at the same time, microplastics are degraded along with
microorganisms, for example, in bioreactors or deposited together in the filtration process.
These results are in line with research conducted by Kataoka et al. [53], which showed
that biochemical oxygen demand as wastewater quality has a positive relationship with
microplastic removal and suspended solid particles in the wastewater. Peller et al. [54]
Water 2022, 14, 2404
number of microorganisms and microplastics in water bodies. When the wastewater
ment unit removes organic elements, at the same time, microplastics are degraded a
with microorganisms, for example, in bioreactors or deposited together in the filtr
process. These results are in line with research conducted by Kataoka et al. [53], w
showed that biochemical oxygen demand as wastewater quality has a 10
positive
rela
of 14
ship with microplastic removal and suspended solid particles in the wastewater. Pel
al. [54] showed that even though microplastic is a part of total suspended solids (
showed
even though
microplastic
is a part
of total suspended
solids
(TSS), there Addit
there
is nothat
obvious
correlation
between
microplastic
and TSS
concentration.
is no obvious correlation between microplastic and TSS concentration. Additional data
data should be collected and analyzed from different weather conditions to justif
should be collected and analyzed from different weather conditions to justify the significant
significant
relationship
between
wastewater
qualityconcentration.
and microplastic concentration.
relationship
between wastewater
quality
and microplastic
Figure
6.6.Correlation
ofmicroplastic
microplastic
concentrations
with
five wastewater
quality parameters.
Figure
Correlation of
concentrations
with five
wastewater
quality parameters.
3.4.
Microplasticpollutant
pollutant load.
concentration
relationship
with biological
organic organ
Microplastic
load. (A)
(A)Microplastic
Microplastic
concentration
relationship
with biological
demand
(BOD); (B)
(B) Microplastic
concentration
relationship
with chemical
oxygen demand
(COD);
demand
(BOD);
Microplastic
concentration
relationship
with chemical
oxygen
demand (C
Microplastic concentration
relationship
with suspended
solids (SS); solids
(D) Microplastic
(C)(C)
Microplastic
concentration
relationship
with suspended
(SS); (D)concentration
Microplastic conc
relationship
with total
phosphorus
(T-P); (E) Microplastic
concentration concentration
relationship with
total
tration
relationship
with
total phosphorus
(T-P); (E) Microplastic
relationship
w
nitrogen (T-N).
total nitrogen (T-N).
This study showed different total numbers of microplastics released from all WWTP.
This study
showed
different in
total
numbers
microplastics
released
from all WW
Although
microplastic
concentration
treated
water isof
low,
considering the
large amount
of
wastewater
discharged
daily,
we
found
that
the
number
of
microplastics
also
released
Although microplastic concentration in treated water is low, considering the large am
along with treated water is very high. Based on this fact, this is in accordance with [4], who
of wastewater
discharged daily, we found that the number of microplastics also rele
stated that WWTP could be considered a point source for releasing microplastics into the
along
with
treated water
is verythat
high.
Based ondischarges
this fact,atthis
is in accordance
wit
aquatic
environment.
Considering
the WWTP–C
the lowest
flow rate
who
stated
that WWTP
could
be considered
a point
source
for releasing
microplastic
with
an average
flow of 8845
m3 /day,
approximately
1.17 billion
microplastics
are emitted
the nearby
river daily, reaching
up to the Nakdong
(Table 2).discharges
The Nakdong
thetoaquatic
environment.
Considering
that theRiver
WWTP–C
atRiver
the lowest
is
one
of
the
most
important
rivers
in
South
Korea.
It
plays
a
significant
role
in
providing
a
3
rate with an average flow of 8845 m /day, approximately 1.17 billion microplastic
water source to metropolitan cities such as Daegu and Busan. It is also a habitat for native
emitted
to the nearby river daily, reaching up to the Nakdong River (Table 2). The
and migratory fish.
dong River is one of the most important rivers in South Korea. It plays a significan
in providing a water source to metropolitan cities such as Daegu and Busan. It is a
habitat for native and migratory fish.
Water 2022, 14, 2404
11 of 14
Table 2. Average microplastic loading amount.
WWTP
Average Number of
Microplastic in the Treated
Water (MP/L)
Average Flow Rate
(m3 /day)
Average Microplastic Released
to the Nearby River
(billion/day)
A
172.5
52,000
8.97
B
90
22,925
2.09
C
32
8845
1.17
A high number of microplastics in treated water released may threaten the aquatic
ecosystem. El Hadri et al., for instance, found that fibers were more toxic to the Ceriodaphnia dubia compared to other shapes [8]. Microplastics can be dangerous if toxic materials
are adsorbed onto the microplastic surface and are eaten by aquatic organisms [55]. Furthermore, aquatic organisms can enter the food chain [56]. Further research is needed to
assess the potential risk of the microplastic released by WWTP to the aquatic ecosystems of
the Nakdong River.
4. Conclusions
The characteristics and removal of microplastics were studied from three full-scale
WWTPs in Gyeonsangbuk-do, South Korea. The results showed the efficiency of the
WWTPs in removing microplastics was high, with a removal rate of >98% from influent
to final effluent. The main proportion of microplastic in all WWTPs were microbeads
and fragments. Microplastic removal mainly occurs in screening, biological treatment,
and sedimentation. Coagulation followed by disc filter showed a better microplastic
removal (WWTP-A and WWTP-B) than only applied disc filter (WWTP-C). Despite the
high efficiency of microplastic removal at each WWTP, many particles escape through the
discharge of treated wastewater. This research proves that microplastics are still found in
the WWTP effluent. Creating new technology and modifying the current WWTP system
for removing microplastics from wastewater is needed in order to reduce the release of
microplastics into the aquatic environment. In addition, it can be used as information input
for environmental authorities in South Korea to improve regulations on plastic waste and
plastic pollution.
Supplementary Materials: The following supporting information can be downloaded at:
https://www.mdpi.com/article/10.3390/w14152404/s1, Figure S1: Average number of microplastics
in different treatment unit; Figure S2. Comparisons removal rate of microplastic and water quality
in Bioreactor and second-ary clarifier. (A) Microplastic removal rate of bioreactor and secondary
clarifier; (B) Removal rate of biological organic demand (BOD); (C) Removal rate of chemical oxygen
demand (COD); (D) Removal rate of suspended solids (SS); (E) Removal rate of total nitrogen (T-N);
(F) Removal rate of total phosphorus (T-P); Table S1: Sampling locations and date; Table S2: Percentages of removal efficiency at each treatment stage for organic material (BOD and COD), suspended
solids (SS), total nitrogen (T-N), and total phosphorus (T-P); Table S3: Overall percentages removal
rate for organic material (BOD and COD), suspended solids (SS), total nitrogen (T-N), and total
phosphorus (T-P).
Author Contributions: Conceptualization, methodology and writing—original draft preparation:
H.J.K.; writing and validation: H.H.; formal analysis: H.H.; review and editing, S.G.P.; supervision,
project administration, T.G.L.; funding acquisition, T.G.L.; All authors have read and agreed to the
published version of the manuscript.
Funding: This research was supported by the Bisa Research Grant of Keimyung University in 2019
(20190682).
Informed Consent Statement: Not applicable.
Data Availability Statement: Not applicable.
Conflicts of Interest: The authors declare no conflict of interest.
Water 2022, 14, 2404
12 of 14
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